We've seen how things can go awry when we tinker with our planet, but geology is fairly resistant to short-term change. When we screw with ecosystems, however, the chain of effects can be long and extremely unpredictable.

Ecosystems are tenuous webs of interconnected species. From the algae at the bottom of the food chain to the apex predators, nothing can be changed without affecting everything else. Underground fungus helps the plants with nitrogen fixing, the plants feed bugs, the bugs feed birds, the birds pollinate the plants, and so on.

Sometimes the effects of ecosystem tinkering are pretty straightforward, like the old lady who swallowed a spider to catch the fly. How's she going to deal with the spider? Last year, conservationists noticed that feral cats were killing the native birds on Macquarie Island, an Australian territory near Antarctica. So they rounded up all the cats and killed them. Problem solved! Except that now the rabbits on the island have no predators left, and left rampant they've obliterated 40 percent of the island's vegetation. This probably doesn't bode well for the birds, either. So the conservationists plan to swallow a bird to catch the spider. That is, they'll be introducing the bane of bunnies, myxomatosis, to the island.

Myxomatosis has its own perils. In 1952, a French scientist decided myxomatosis was the answer to the rabbit problem in his garden, so he injected the disease into a few of them. Two years later, only 10 percent of France's entire wild rabbit population remained. The disease made it to the U.K. not long after with similar effects. Gardeners might not miss a few hundred million rabbits, but critically endangered animals like the Iberian Lynx certainly do.

Then there are those times when ecological change plays out like some kind of biological Rube Goldberg machine. The L.A. Times recently reported the resurgence of wetlands in Yellowstone National Park. The wetlands have returned thanks to wolves. Wolves and wetlands don't really seem related. Allow me to explain in my best James Burke voice.

Wolves, being natural predators, eat elk. In the absence of wolves, elk proliferate substantially. Elk are natural predators, too. It's just that their preferred menu items are the saplings of willow and aspen trees. A dearth of mature willows and aspens leaves a gap in the diet of this fellow…castor canadensis, the North American beaver. Worse, it gives him nothing to build lodges and dams out of. Those dams, of course, block streams, recharge springs, create wetlands and help generally keep things wet throughout the year. Bring back the wolves and you reduce the elk, allowing willows to mature, letting the beavers build their dams, and regenerating the wetlands. So you see? It's very simple.

The examples could go on forever, but the fact is this – whenever we've tried to change an ecosystem, we've made a mess of it, virtually without exception. And that's not even counting the number of times we've screwed things up accidentally, like carrying zebra mussels into North American waters. That leads us to the chief question for the fourth and final part of this series: given our frequent failures to terraform our own planet without disastrous consequences, do we have the right to do it to other planets?